1. Field of the Invention
This invention relates to a programmable ROM for electrically storing data. More particularly, the invention relates to a programmable ROM whose memory cells are floating gate type MOS transistors, and which uses a high voltage to write data into the memory cells, and a low voltage to read data from the memory cells.
2. Description of the Related Art
In general, erasable programmable ROMs (EPROMs) employ floating gate type MOS transistors as memory cells. When such EPROMs are in a read mode, a read voltage of, for example, 5V, is applied to the control gate of the cell transistor, while when in a write mode or a program mode, a write voltage of, for example, 12.5 V, is applied to the control gate and drain of the cell transistor.
The reason for EPROMs needing different voltages for reading data from, and writing data into, the memory cells is as follows:
A programmable ROM has two power source terminals. The first power source terminal continuously receives an external power source voltage of 5 V, and a second power source terminal receives a power source voltage of 5 V when in a read mode, but one of 12.5 V when in a write mode.
The programmable ROM includes a circuit designed for only data-reading operation, such as a sense amplifier, a data output buffer, or the like, and a circuit designed for both data-reading operation and programming operation, such as a column decoder, a row decoder, or the like. The data-reading circuit comprises transistors having a short channel, so that the circuit may operate at a high speed. These short channel type transistors cannot withstand the power source voltage of 12.5 V. Therefore, the power source voltage of 5 V is applied to these transistors, thereby to drive them.
The data-reading and programming circuit has an output buffer section which comprises transistors having a long channel since the circuit must apply a high voltage to the memory cells of the programmable ROM. The output buffer section of the circuit is connected to the second power source terminal. This section is driven by the power source voltage of 5V to read data from the ROM, and by the power source voltage of 12.5 V to write data into the ROM. The other sections of the circuit comprise transistors having a short channel, and are driven by the power source voltage of 5 V.
As has been described, the programmable ROM requires two types of power source voltages, which are applied to the first and second power source terminals, respectively. Therefore, the IC package of this ROM needs an earth pin for earth potential (0 V) and two pins used as the first and second power source terminals. In general, the more pins an IC package has, the greater area the package occupies on the printed circuit board. Hence, the greater the area occupied by the package, the lower the packing density on the board. In other words, the less pins, the better.
The conventional programmable ROM requires more pins than other IC memory devices, such as ordinary ROMs and RAMs. More specifically, it must have a 5V/12.5V power source pin, in addition to those pins which the ordinary ROM and the RAM have, such as address pins, I/O pins, control pins, a ground pin, a 5V power source pin.